Don't waste RAM in the low-level TWI Wire drivers.

Since twi_readFrom() and twi_writeTo() are both blocking functions,
there is no need to allocate a special twi_masterBuffer.  Doing so
wastes valuable RAM, uses extra time to copy the data to the secondary
buffer, and limits the transfer size to TWI_BUFFER_SIZE.  Instead, it
only needs a pointer to the buffer for the IRQ to use for the transfer.
And, if asynchronous non-blocking functions are ever added, which will
require a different API and callbacks, etc., then the existing txBuffer
and rxBuffer for slave mode can just be used there too, since master
mode and slave mode can't both be active at the same time.

Author: dewhisna

avr/libraries/Wire/src/utility/twi.c

@@ -59,7 +59,7 @@ static volatile bool twi_do_reset_on_timeout = false;  // reset the TWI register
 static void (*twi_onSlaveTransmit)(void);
 static void (*twi_onSlaveReceive)(uint8_t*, int);
 
-static uint8_t twi_masterBuffer[TWI_BUFFER_SIZE];
+static uint8_t *twi_masterBuffer;
 static volatile uint8_t twi_masterBufferIndex;
 static volatile uint8_t twi_masterBufferLength;
 
@@ -159,8 +159,6 @@ void twi_setFrequency(uint32_t frequency)
  */
 uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length, uint8_t sendStop)
 {
-  uint8_t i;
-
   // ensure data will fit into buffer
   if(TWI_BUFFER_SIZE < length){
     return 0;
@@ -186,6 +184,7 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length, uint8_t sen
   twi_error = 0xFF;
 
   // initialize buffer iteration vars
+  twi_masterBuffer = data;
   twi_masterBufferIndex = 0;
   twi_masterBufferLength = length-1;  // This is not intuitive, read on...
   // On receive, the previously configured ACK/NACK setting is transmitted in
@@ -244,11 +243,6 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length, uint8_t sen
   if (twi_masterBufferIndex < length)
     length = twi_masterBufferIndex;
 
-  // copy twi buffer to data
-  for(i = 0; i < length; ++i){
-    data[i] = twi_masterBuffer[i];
-  }
-
   return length;
 }
 
@@ -270,8 +264,6 @@ uint8_t twi_readFrom(uint8_t address, uint8_t* data, uint8_t length, uint8_t sen
  */
 uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait, uint8_t sendStop)
 {
-  uint8_t i;
-
   // ensure data will fit into buffer
   if(TWI_BUFFER_SIZE < length){
     return 1;
@@ -298,14 +290,10 @@ uint8_t twi_writeTo(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait
   twi_error = 0xFF;
 
   // initialize buffer iteration vars
+  twi_masterBuffer = data;
   twi_masterBufferIndex = 0;
   twi_masterBufferLength = length;
 
-  // copy data to twi buffer
-  for(i = 0; i < length; ++i){
-    twi_masterBuffer[i] = data[i];
-  }
-
   // build sla+w, slave device address + w bit
   twi_slarw = TW_WRITE;
   twi_slarw |= address << 1;

avr/libraries/Wire1/src/utility/twi1.c

@@ -46,7 +46,7 @@ static volatile uint8_t twi_inRepStart; // in the middle of a repeated start
 static void (*twi_onSlaveTransmit)(void);
 static void (*twi_onSlaveReceive)(uint8_t*, int);
 
-static uint8_t twi_masterBuffer[TWI1_BUFFER_SIZE];
+static uint8_t *twi_masterBuffer;
 static volatile uint8_t twi_masterBufferIndex;
 static volatile uint8_t twi_masterBufferLength;
 
@@ -146,8 +146,6 @@ void twi_setFrequency1(uint32_t frequency)
  */
 uint8_t twi_readFrom1(uint8_t address, uint8_t* data, uint8_t length, uint8_t sendStop)
 {
-  uint8_t i;
-
   // ensure data will fit into buffer
   if(TWI1_BUFFER_SIZE < length){
     return 0;
@@ -163,6 +161,7 @@ uint8_t twi_readFrom1(uint8_t address, uint8_t* data, uint8_t length, uint8_t se
   twi_error = 0xFF;
 
   // initialize buffer iteration vars
+  twi_masterBuffer = data;
   twi_masterBufferIndex = 0;
   twi_masterBufferLength = length-1;  // This is not intuitive, read on...
   // On receive, the previously configured ACK/NACK setting is transmitted in
@@ -200,11 +199,6 @@ uint8_t twi_readFrom1(uint8_t address, uint8_t* data, uint8_t length, uint8_t se
   if (twi_masterBufferIndex < length)
     length = twi_masterBufferIndex;
 
-  // copy twi buffer to data
-  for(i = 0; i < length; ++i){
-    data[i] = twi_masterBuffer[i];
-  }
-
   return length;
 }
 
@@ -225,8 +219,6 @@ uint8_t twi_readFrom1(uint8_t address, uint8_t* data, uint8_t length, uint8_t se
  */
 uint8_t twi_writeTo1(uint8_t address, uint8_t* data, uint8_t length, uint8_t wait, uint8_t sendStop)
 {
-  uint8_t i;
-
   // ensure data will fit into buffer
   if(TWI1_BUFFER_SIZE < length){
     return 1;
@@ -242,14 +234,10 @@ uint8_t twi_writeTo1(uint8_t address, uint8_t* data, uint8_t length, uint8_t wai
   twi_error = 0xFF;
 
   // initialize buffer iteration vars
+  twi_masterBuffer = data;
   twi_masterBufferIndex = 0;
   twi_masterBufferLength = length;
 
-  // copy data to twi buffer
-  for(i = 0; i < length; ++i){
-    twi_masterBuffer[i] = data[i];
-  }
-  
   // build sla+w, slave device address + w bit
   twi_slarw = TW_WRITE;
   twi_slarw |= address << 1;